It is known to manufacture a vehicle frame by providing separate subassemblies, each subassembly being composed of several separate components that can include lineal frame members. Each subassembly is manufactured by joining together several members by means of a node structure that can be a cast, extruded, or sheet component. The frames and subassemblies can be assembled by adhesive bonding, welding, or mechanical fastening; or by combinations of these and other joining techniques. An example of such a vehicle frame structure is available in U.S. Pat. No. 4,618,163, entitled "Automotive Chassis" the entire contents of which are incorporated herein by reference. Aluminum alloys are highly desirable for such vehicle frame constructions because they offer low density, good strength and corrosion resistance. Moreover, aluminum alloys can be employed to improve the vehicle frame stiffness and performance characteristics. Use of aluminum provides the potential for environmental benefits and efficiencies through a lightweight aluminum vehicle frame that also demonstrates reduced fuel consumption due to the lightweighting. Finally, the application of aluminum alloy components in a vehicle frames presents an opportunity to ultimately recycle the aluminum components/subassemblies when the useful life of the vehicle is spent. Moreover, it is believed that an aluminum vehicle frame retains the perceived strength and crashworthiness typically associated with much heavier, conventional steel frame vehicle designs.
As suggested above important considerations for aluminum primary automotive body structures include crashworthiness in conjunction with reducing the overall vehicle weight and/or improving vehicle performance. For the automotive application, crashworthiness reflects the ability of a vehicle to sustain some amount of collision impact without incurring unacceptable distortion of the passenger compartment or undue deceleration of the occupants. Upon impact, the structure should deform in a prescribed manner; the energy of deformation absorbed by the structure should balance the kinetic energy of impact; the integrity of the passenger compartment should be maintained; and the primary structure should crush in such a manner as to minimize the occupant deceleration. Various standard tests can be used to evaluate the physical and mechanical properties of an aluminum alloy for use in an automotive structure or other applications. As examples, tensile testing and standard formability tests can be used to provide information on strength and relative performance expectations, or a tear test can be used to examine fracture characteristics and provide a measure of the resistance to crack growth or toughness under either elastic or plastic stresses. These and other test methods are used to examine the general performance of materials representative of those used for the manufacture of vehicle components, subassemblies, and frames. However, few standard tests are available to allow the evaluation of aluminum alloy components intended for use in primary body structures. Accordingly, in addition to the tests described above, it is believed that a static axial crush test allows the evaluation of the response of a vehicle frame component to axial compressive loading. If used for evaluation of component geometries designed to provide absorb energy under compressive loading, the static axial crush test provides the severe conditions necessary to examine a component's response to compressive loading. During the static axial crush test, a specified length of an energy absorbing component is compressively loaded at a predetermined rate creating a final deformed component height of approximately half the original free length or less. Various modes of collapse can be experienced under these conditions; including: regular folding--stable collapse, irregular folding, and bending. The desired response for evaluation of energy absorbing components is stable axial collapse characterized by regular folding. The crushed sample is examined to determine material response to the severe deformation created during this test. It is generally desirable to demonstrate the ability to deform without cracking. In this case, samples are visually examined following static axial crush testing and assigned a rating based on the appearance of the deformed samples. The results of the examination are registered on a scale of from 1 to 3. A "3" indicates that the area proximate the fold shows evidence of open cracking that is often visible to the naked eye and roughening damage. A "3" rated material is considered to be unacceptable. A " 2" indicates that the area proximate the folds or displaced side wall material of the extrusion is roughened and may be slightly cracked, but the basic integrity of the side wall is maintained. A sample rated "2" is better than one rated a "3" but not as good as a sample rated "1". A rating of "1" indicates that the crushed extrusion contains no cracking or roughened areas and the folds are substantially smooth; this is the preferred material response following the static axial crush test.
The ability of a structure or structural component to absorb energy and deform in a desired, progressive manner under compressive loading during both static and dynamic crash testing is a function of both the component design, e.g., geometry, cross-section shape, size, length, thickness, joint types included in the assembly, and the properties of the material from which the component is manufactured, i.e., yield and ultimate tensile strength at the actual loading rate, modulus of elasticity, fracture behavior, etc. Various aluminum alloys are potential candidates for the manufacture of a primary body structure which includes such energy absorbing components. For example, 6XXX alloys, could be utilized in the production of extruded components for incorporation into aluminum intensive vehicles. The 6XXX series alloys are a popular family of aluminum alloys, designated as such in accordance with the Aluminum Association system wherein the 6XXX series refers to heat treatable aluminum alloys containing magnesium and silicon as their major alloying additions. Strengthening in the 6XXX alloys is accomplished through precipitation of Mg2Si or its precursors. The 6XXX are widely used in either the naturally aged-T4 or artificially aged-T6 tempers. This series of alloys also commonly includes other elements such as chromium, manganese, or copper, or combinations of these and other elements for purposes of forming additional phases or modifying the strengthening phase to provide improved property combinations.
The 6XXX alloys are commonly used for production of architectural shapes, and because these products are most often used in applications requiring only a minimum strength level the 6XXX alloys typically are air quenched in production due to cost considerations. Alloy 6063 represents one of the most widely used 6XXX products. It provides typical yield strengths of 90 MPa [13 ksi], 145 MPa [21 ksi], and 215 MPa [31 ksi] in the naturally aged-T4 and artificially aged-T5 and -T6 tempers, respectively. By accepted industry convention, both the -T5 and -T6 temper designations for extrusions can refer to a product which has been press quenched and artificially aged in lieu of the strict definition of -T6 that includes a solution heat treatment and quenching operation.
Quenching from elevated temperature processing operations is often critical to the development of properties and performance required of the final product. The objective of quenching is to retain the Mg, Si and other elements in the solid solution resulting from an elevated temperature operation such as extrusion. In the case of extrusion, the product is often quenched as it exits the extrusion press to avoid the additional cost associated with a separate solution heat treatment and quenching operation. Water quenching can be used to provide a fast cooling rate from the extrusion temperature. A fast cooling rate provides the best retention of the elements in solid solution. However water quenching creates the need for additional equipment and can create excessive distortion and the need for subsequent processing to correct the shape prior to use. Air cooling is commonly used for press quenching of 6063 products. Air cooling reduces the distortion experienced and improves dimensional capability in hollow products. However, 6XXX products typically exhibit some quench sensitivity or loss of strength or other properties with reduced quench rates experienced in air quenching. Quench sensitivity is due to precipitation of elements from the solid solution during a slow quench. This precipitation typically occurs on grain boundaries and other heterogeneous sites in the microstructure. Precipitation during the quenching operation makes the solute unavailable for precipitation of strengthening phases during subsequent aging operations. A slow quench typically results in a loss of strength, toughness, formability or corrosion resistance. A slow quench can also adversely effect the fracture performance of the product by promoting low energy grain boundary fracture. Quench sensitivity with respect to yield strength is generally small in dilute alloys such as 6063. However, pronounced quench sensitivity can be observed with respect to toughness and toughness indicators as well as other properties which are strongly influenced by the fracture behavior of the material. Differences are often noted in the results obtained through tear tests, and formability evaluations. Dramatic influences of the quench rate have also been noted in the results obtained using the static axial crush test in common commercial extrusion materials such as 6063. In order to overcome the loss of desired properties, a separate solution heat treatment and quench, or an in line press spray water quench can be used to provide cooling at the required rate to minimize precipitation during quenching. However, as indicated above, water quenching can create distortion, inhibit process speed, require additional processing for dimensional correction, and limit the ability to produce component profiles to tolerance. The strictest of tolerances must be maintained during the assembly of a vehicle subassembly or frame. Quench distortion associated with use of a water quenching operation adversely effects the production of a complex, thin walled, hollow extrusion, potentially distorting it and rendering it out of tolerance for the desired application and in need of further labor intensive correction.
U.S. Pat. No. 4,525,326 teaches that the quench sensitivity with respect to strength of a 6XXX alloy (Si, Fe, Cu, Mg) can be improved by the addition of vanadium. Specifically, the patent discloses the addition of 0.05 to 0.2% vanadium and manganese in a concentration equal to 1/4 to 2/3 of the iron concentration to an aluminum alloy for the manufacture of extruded products. Notwithstanding such efforts to develop alloys that offer reduced quench sensitivity with respect to strength; there remains a need for alloys that provide reduced quench sensitivity with respect to static axial crush performance.
An alloy that could be air quenched would provide the ability to produce thin walled hollow extruded shapes meeting the dimensional capabilities desired for assembly of automotive structures and providing the characteristics desired for use in the final structure including good strength and the ability to deform in a regular way in components designed to absorb energy when compressively loaded in the event of a collision; and allow production of these components in a cost effective manner.
It is an object of this invention to provide an aluminum alloy component characterized by excellent static axial crush performance.
It is another object of the invention to provide an aluminum alloy characterized by reduced quench sensitivity with respect to static axial crush performance and other characteristics required for application in automotive structures.
It is also an object of this invention to provide an aluminum alloy capable of an increased range of shapes including thin walled hollow extrusions and improved dimensional capability for use in the manufacture of aluminum intensive vehicles or similar structures.
It is a further object of this invention to provide an improved aluminum alloy.
It is yet an object of this invention to provide a method of manufacturing an improved elongated aluminum alloy product.